Many unexplained strandings and deaths of marine mammals could be caused by soundwaves from underwater military sonar equipment, zoologists believe.

They think the sonar signals may cause bubbles in the animals' tissue, in much the same way as divers can suffer decompression sickness known as "the bends".

Writing in the journal Nature, scientists describe how 14 whales died during a naval exercise in the Canary Islands.

They say sonar use may need to be regulated to protect the mammals.

The link they suggest is not new: environment groups have argued for years that sonar is a threat to cetaceans (whales, dolphins and porpoises).

Two months ago the US Navy was ordered by a federal court not to test a powerful sonar system in most parts of the world, because of fears the booming sounds emitted to detect enemy submarines could "irreparably harm" cetaceans and fish.

But the scientists, from the UK and Spain, have broken new ground by discovering damage to the livers and kidneys of animals they examined, including gas-filled cavities which they say are new to marine mammal pathology.

They say the bubbles they found in the animals' tissues resemble those found in divers affected by decompression sickness (DCS).

Suffering stopped

They outline the circumstances surrounding the deaths in September 2002 of 14 beaked whales during a Spanish-led international naval exercise in the Canaries.

About four hours after "the onset of mid-frequency sonar activity" all 14 stranded themselves and then died on the beaches of Fuerteventura and Lanzarote.

The exercise was abandoned

The Spanish navy then brought the exercise to an end to prevent risking further harm to the cetaceans.

A team of Spanish scientists carried out autopsies on eight Cuvier's beaked whales, one Blainville's and one Gervais' beaked whale. All showed damage consistent with DCS.

The Nature report also details the results of autopsies performed on cetaceans stranded in the UK between October 1992 and January 2003 - three common and three Risso's dolphins, one harbour porpoise, and a Blainville's beaked whale.

A team from the UK Marine Mammals Stranding Project found gas bubbles in their blood vessels, and haemorrhages in internal organs, characteristic indications of DCS.

The project is being co-ordinated by the Institute of Zoology, part of the Zoological Society of London.

Deep divers in danger

Dr Paul Jepson of the institute said: "We discovered that a small number of stranded animals had gas bubbles and associated tissue injuries.

Evidence came from autopsies

"Although DCS was previously unheard-of in marine mammals, we concluded that a form of marine mammal DCS was the most likely cause.

"This new evidence challenges the widely held notion that cetaceans cannot suffer from decompression sickness."

"Their whole diving profile - not just the actual depth they reach - means they absorb more nitrogen into their tissues, and are therefore more at risk when they surface.

Mechanism unclear

"Beaked whales have the highest nitrogen levels, and so they are at the highest risk.

"Baleen whales don't normally dive as deep, but sperm whales do, and they could possibly face a similar danger."

What is still unclear is whether the sonar could be damaging the cetaceans directly by somehow affecting their tissues, or whether the soundwaves frighten them into making too rapid an ascent, with the same result.

The researchers will now be trying to establish what level of sound can induce this effect in cetaceans exposed to sonar.